How well do stormwater wetlands remove pollutants? 

Over 10 stormwater wetlands in NC have been monitored to study pollutant removal.  Removal of the primary stormwater pollutants of concern in North Carolina– sediment, nitrogen, and phosphorus, from several stormwater wetland sites are summarized here:

In North Carolina, stormwater wetlands are given credits of 85%, 30% and 40% for TSS, TP, and TN removal. 
Stormwater wetlands may also reduce fecal coliform and other indicator bacteria, though bacteria removal by wetlands is likely limited by intermal production by resident animals and bacterial persistance in the moist environment afforded by wetlands.

Can pollutant removal performance be improved by placing stormwater wetlands in series? 

• Stormwater wetlands placed in series should not receive the same removal credit as concentration reductions diminish between successive cells.
• There are limits to pollutant removal in natural-based systems such as stormwater wetlands, particularly organic nitrogen. 

Do wetlands harbor mosquitoes? 

Mosquitoes, which spend the first part of their life cycle in water, are a legitimate concern in stormwater wetlands and other stormwater structures that provide an open water habitat.  Aquatic insect and mosquito larvae sampling has been conducted in stormwater wetlands and ponds across the state of North Carolina in 2004 and 2009 to determine if mosquitoes are an issue in these systems and, if so, what factors are associated with their presence.  Results from this research include:

• Mosquito larvae were found in about one-third of wetland sites.
• Mosquito larvae were more likely to be found in wetlands with large expanses of cattails and willows.
• Mosquito larvae were less likely to be found in wetlands that also supported fish, particularly mosquito fish (Gambusia spp.).
• Mosquito larvae were less likely to be found in stormwater wetlands, which supported a greater abundance of dragonfly larvae and other predatory insects, than in stormwater ponds, which supported a smaller abundance of predatory insects overall.

This research indicates that predators – both fish and predatory aquatic insects - are key in controlling mosquito populations in stormwater wetlands.  Mosquito predators will naturally colonize stormwater wetlands, but their presence can be further encouraged with the following design and maintenance guidelines:

• Select flowering wetland plants (such as Pickerel Weed) to attract adult dragon and damselflies, the larvae of which are voracious predators.  These insects are thought to use such plants as visual cues to lay eggs, with some species even laying their eggs in the plant.
• Improve predator access to a greater portion of the wetland by distributing pools throughout.
• Prevent the development of extensive expanses of cattails or willows to limit the formation of areas for mosquito breeding that are inaccessible to other predators.